The use of semiconductor devices and integrated electronic circuitry has brought significant reductions in the size, weight and cost of audio amplifier circuitry as compared with discrete electronics approaches of a prior generation. In attempts to exploit the potential of semiconductor devices and integrated electronics, audio engineers have endeavored to develop amplifiers with increased audio output power in a smaller, lighter, more cost-effective package while maintaining high quality sound reproduction. As the size, weight and cost of integrated electronics have decreased, audio amplifier packaging form factors have come to be dominated by the size and weight of bulky power transformers and associated heat sinking conventionally used to provide the electronic circuitry with DC power from an alternating current (AC) power source. In conventional power supply designs for audio amplifiers, two DC bias levels and a ground reference are generated from the AC line power source commonly available from the wall sockets of buildings and homes. Transformer physical parameters have thus imposed a limitation on the extent to which audio power amplifiers may be miniaturized with conventional audio amplifier circuit topologies powered by conventional power supplies.
Audio engineers have employed a number of techniques in attempts to overcome or mitigate the size and weight constraints imposed by conventional power supply circuitry. In U.S. Pat. No. 4,484,150, an apparatus is described which uses pulsed power techniques to enable the use of a much smaller power transformer for a given audio power output and for a given alternating current power input frequency. Other approaches to minimize transformer size and weight have employed switching power supplies operating at very high switching frequencies (e.g., 20,000 Hz). As the operating frequency of such switching power supplies increases, the number of turns in the coils of the transformer can be reduced correspondingly for the same amount of transformer power output and transformer size may thus be decreased.
A further improvement in the design of switching power supplies for the reduction of power transformer size and weight is disclosed in U.S. Pat. No. 4,808,946, which discloses power pulse width modulation in a switching power supply in order to match the power delivered through the transformer to the instantaneous power needs of the audio amplifier circuit. In that approach, load current sense feedback is provided to the pulse width modulation circuitry to control the power pulses delivered through the transformer.
While the aforementioned design approaches can to some extent mitigate the size, weight and cost problems of conventional transformer-based audio amplifier power supplies, and while for certain applications and design objectives these approaches may be desirable, there are accompanying design problems which offset to some extent the advantages derived from the use of these approaches. For example, circuit complexity and costs may be increased substantially with such approaches relative to transformer-based designs. Circuit designs requiring two DC bias levels and a ground reference thus suffer from inherent limitations which impose constraints on the extent to which transformer size and weight, and hence circuit size, weight and cost, may be minimized.